2020-21 Rocky Mountain Mechanics Seminar Series

 
Control of mechanical wave propagation is key to Air Force applications in vibration damping, battlefield acoustics, non-destructive evaluation, ultrasonic imaging, impact resistance and aeroacoustics.  Many of the commercial solutions available in these application areas are parasitic in that they are added after structural performance optimization, they are not in a form factor that is desirable for the U.S. Air Force/Space Force, or they are too large and massive to be used on an aircraft. However, resonant metamaterials have emerged as a promising material set to overcome many of these problems. Emerging additive manufacturing technologies coupled with material innovations and a fundamental understanding of the dynamics of a structure have spurred the development of new metamaterial solutions for many Air Force problems. This talk will provide an overview of a few of the research projects on resonant metamaterials in the Materials and Manufacturing Directorate of AFRL including projects on low frequency sound mitigation, low frequency vibration control, ultrasonic imaging, and controlling instabilities in aerodynamic flow.

About Abigail Juhl
Air Force Research Laboratory

Dr. Abigail Juhl is a Materials Research Engineer in the Materials and Manufacturing Directorate at Air Force Research Laboratory. Dr. Juhl is leading an effort to dynamically control mechanical wave propegation through architected materials for application in acoustic and vibration mitigation. She is an expert in patterning nano- and micro-scale materials over macroscale volumes. Juhl received her bachelor's of science in Materials Science and Engineering from North Carolina State University and her doctorate in Materials Science and Engineering from the University of Illinois Urbana-Champaign. She completed a National Research Council Postdoctoral Fellowship in the Optical Materials Branch at AFRL before starting in her current position. She also served as the Acting Assistant Chief Scientist and the Polymers and Responsive Materials Research Team Lead for the Materials and Manufacturing Directorate. She won the AFRL Early Career Award and DOD Lab Scientist of the Quarter in 2020.

It used to be that academic seminars involved a researcher visiting a host institution to deliver a talk and meet with colleagues (do you remember those days?!). Times have changed, at least temporarily, but this situation is also opening opportunities. In this "talk," we will be inviting you for a virtual tour of our Flexible Structures Laboratory (fleXLab) at EPFL in Switzerland. We will show you some of our experimental facilities and share some of our recent research activities, focusing on the mechanics of magneto‐active structures. Multiple members of our team will be involved in this tour. Research at our fleXLab focuses is centered in the general area of the mechanics of slender structures, which leverage their postbuckling regime for novel modes of functionality. Methodologically, we recognize scaled high‐precision model experiments as a powerful tool for discovery in mechanics, supported by theory and computation, in a vision of science‐enable engineering and engineering‐motivated science. Recently, we have become fascinated with active structures made out of magneto‐rheological elastomers that can be actuated in the presence of an external magnetic field. After introducing some recent advances in experimentation, modeling, and computational for this class of systems, we will present a series of concrete examples. Specifically, we will discuss (i) (re)programmable mechanical metamaterials with programmable memory; (ii) magneto‐active beams and Kirchhofflike rods; and (iii) magnetic shells with tunable buckling properties. This virtual lab will involve the participation of the following fleXLab members: Tim Chen (postdoc), Arefeh Abbasi (PhD student), and Dong Yan (postdoc). We are looking forward to "e‐hosting" you at our fleXLab at EPFL.

About Pedro M. Reis
Institute of Mechanical Engineering - École Polytechnique Fédérale de Lausanne (EPFL), Switzerland

Pedro Miguel Reis is a Professor of Mechanical Engineering at the École Polytechnique Fédérale de Lausanne in Switzerland, where he is the director of the Institute of Mechanical Engineering. Professor Reis received a B.Sc. in Physics from the University of Manchester, UK (1999), a Certificate of Advanced Studies in Mathematics (Part III Maths) from St. John’s College and DAMTP, University of Cambridge (2000) and a Ph.D. in physics from the University of Manchester (2004). He was a postdoc at the City College of New York (2004‐2005) and at the CNRS/ESPCI in Paris (2005‐2007). He joined MIT in 2007 as an Instructor in Applied Mathematics. In 2010 he moved to MIT’s School of Engineering, with dual appointments in Mechanical Engineering and Civil & Environmental Engineering, first as the Esther and Harold E. Edgerton Assistant Professor and, since the summer of 2014 as Gilbert W. Winslow Associate Professor. In October 2013, the Popular Science magazine named Professor Reis to its 2013 “Brilliant 10” list of young stars in Science and Technology. He has also received the 2014 CAREER Award (NSF), the 2016 Thomas J.R. Hughes Young Investigator Award (Applied Mechanics Division of the ASME), the 2016 GSOFT Early Career Award for Soft Matter Research (APS), he is a Fellow of the APS, and he is the 2021 President of the Society of Engineering Science (SES).

The fascinating diversity of material behavior at the macroscopic scale can only emerge from the underlying atomistic or particle behavior. Yet, the direct connection between these two scales remains an extremely challenging quest, particularly in the context of non-equilibrium phenomena. In this talk, we will discuss several advances in this direction, in the context of plasticity, thermoelasticity, diffusion and viscous dissipation. In all these cases, the importance of fluctuations in the effective response will become apparent. More precisely, these will provide crucial information for the material description and evolution at the continuum scale, where the behavior is modeled as deterministic and free of fluctuations.

About Celia Reina
University of Pennsylvania

Celia Reina is the William K. Gemmill Term Assistant Professor in Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. She joined in 2014 after holding the Lawrence Postdoctoral Fellowship at Lawrence Livermore National Laboratory and the HCM postdoctoral fellowship at the Hausdorff Center of Mathematics in Bonn, Germany. Dr. Reina received her PhD from the California Institute of Technology in Aerospace Engineering in 2011, under the supervision of Prof. Michael Ortiz, following a BS in Mechanical Engineering from the University of Seville in Spain and a Master in Structural Dynamics from Ecole Centrale Paris in France. She is the 2017 recipient of the Eshelby Mechanics Award for Young Faculty and the 2021 recipient of an NSF CAREER award.

The term tensegrity, derived from tensional integrity, refers to a certain class of structural systems composed of bars and strings. Through adequate pre-stressing of their string members, tensegrity structures generally become mechanically stable. Traditional approaches for modeling their behavior assume that (i) bars are perfectly rigid, (ii) cables are linear elastic, and (iii) bars experience pure compression and strings pure tension. In addition, a common design constraint is to assume that the structure would fail whenever any of its bars reaches the corresponding Euler buckling load. In reality, these assumptions tend to break down in the presence of dynamic events. In the first part of this talk, we will introduce a physics-based reduced-order model to study aspects related to the dynamic and nonlinear response of tensegrity-based planetary landers. We will then adopt our model to show how, under dynamic events, buckling of individual members of a tensegrity structure does not necessarily imply structural failure, thus significantly expanding the design space for such vehicles. In the second part of this talk, we will show how lessons learned from our tensegrity planetary lander can be translated into to the development novel metamaterials. We will introduce the first known class-two 3D tensegrity metamaterial, and show that this new topology exhibits unprecedented static and dynamic mechanical properties.

About Julián J. Rímoli
Georgia Institute of Technology

Julián J. Rimoli the Pratt & Whitney Associate Professor of Aerospace Engineering at the Georgia Institute of Technology. He obtained his engineering diploma in aeronautics from Universidad Nacional de La Plata in 2001. In 2004 he moved to the United States to pursue graduate studies at Caltech, receiving his MSc and PhD in aeronautics in 2005 and 2009 respectively. Upon graduation Dr. Rimoli accepted a postdoctoral associate position at the Department of Aeronautics and Astronautics of MIT, where he performed research for over a year and a half. He joined Georgia Tech in 2011. His research interests lie within the broad field of computational mechanics of materials and structures, with special interest in problems involving multiple length and time scales, and in the development of theories and computational techniques for seamlessly bridging them. He is an associate fellow of AIAA, and is the recipient of the NSF CAREER Award, the Donald W. Douglas Prize Fellowship, the Ernest E. Sechler Memorial Award in Aeronautics, the James Clerk Maxwell Young Writers Prize, the Lockheed Dean's Award for Excellence in Teaching, the Class of 1940 Teaching Effectiveness Award, and the Goizueta Junior Faculty Professorship.

By endowing homogeneous materials with architecture, the material property space can be expanded. In this talk I will focus on dense architecture materials obtained by segmentation of the monolithic system into topologically interlocking building blocks. First, I will demonstrate the conceptual advantages of such an approach using a canonical model, which I will also use to show the underlying mechanics principles. I then proceed to investigate stereotomic material systems with architectures derived from Archimedean and Laves tessellation, as well as from distorted square tessellation. I will discuss and analyze the relationships between architecture and mechanical properties. I will conclude with a discussion on potential applications to technology relevant problems.

About Thomas Siegmund
Purdue University

Thomas Siegmund is professor of mechanical engineering at Purdue University. His research group investigates the fundamental question on strength and toughness. He applies finding to technologically relevant problems in wide array of applications. Thomas Siegmund served as the president of the Society of Engineering Science, 2017, and as the NSF Program Director for Mechanics of Materials and Structures (2013-2015).

Robotic surfaces that can reshape and react to external stimuli offer opportunities to create soft, versatile machines that can multi-task while interacting safely with their surroundings. Such systems are useful in applications that range from haptic interfaces, wearable exoskeletons and reconfigurable medical supports. Key properties of robotic surfaces include their ability to control their local stiffness, reprogram their target shape and have sufficient mechanical loadbearing ability, to support weights and manipulate objects. In this talk, I will describe recent solutions developed in our group, to create structured fabrics that have tunable bending stiffness and robotic surfaces that allow for large, reprogrammable, and pliable shape morphing into smooth 3D geometries. To develop these solutions, we design layered, architected materials, consisting of interlocking particles or networks of layered, polymeric ribbons. We employ different actuation methods, including vacuum pressure and Joule heating, to control the response of the surfaces.  We demonstrate the ability to fabricate fabrics that become >25 times stiffer than their relaxed configuration, when a small external pressure (~93 kPa) is applied. We also show that robotic surfaces consisting of layers of heat responsive liquid crystal elastomers (LCEs) can be reprogrammed to assume arbitrary shapes. 

About Chiara Daraio
California Institute of Technology

Chiara Daraio is the G. Bradford Jones Professor of Mechanical Engineering and Applied Physics at Caltech. She received her undergraduate degree in mechanical engineering from the Universita Politecnica delle Marche, Italy (2001) and her MS (2003) and PhD degrees (2006) in materials science and engineering from the University of California, San Diego. She joined the aeronautics and applied physics departments of the California Institute of Technology in fall of 2006 and was promoted to full professor in 2010. Between 2013-2016, she served as the chair of mechanics and materials at ETH Zürich. 

Chiara received numerous awards, among them, a Presidential Early Career Award from President Obama (PECASE) and an ONR Young Investigator Award. She was selected as a Sloan Research Fellow and she is a winner of the NSF CAREER award, of the Richard Von Mises Prize and of the Hetenyi Award from the Society for Experimental Mechanics. She was nominated by Popular Science magazine among the "Brilliant 10." She serves as a board editor for science (AAAS) and as an associate editor for the journals Multifunctional Materials (IOP), Matter (Cell Press) and Frontiers in Materials (Frontiers).

We discuss recent advances in developing a fundamental, mechanistic understanding of the evolution of surface roughness of solids during dry sliding. In the first part, we summarize our attempts at capturing debris formation at micro contacts using atomistic potentials [1,2]. We show that, in the simple situation of an isolated micro contact, the final debris size scales with the maximum junction size attained upon shear. This permits to draw analogies with Archard adhesive wear model [3]. In the second part, this single-asperity understanding is incorporated in a mesoscale model [4], which aims at estimating from first principles the wear coefficient, a notoriously little understood parameter in wear models. We estimate the amount of volume of debris formed for a given applied load, using the probability density of micro contact sizes. A crucial element of this mesoscale model is the distribution of surface heights, which should evolve as wear processes take place. This leads us, in the final part, to a discussion of recent simulations aiming at understanding the long term evolution of surface roughness. These long time scales simulations reveal the emergence of self-affine fractal surfaces irrespective of the initial roughness [5].

About Jean-Francois Molinari
Computational Solid Mechanics Laboratory, Civil Engineering Institute, Materials Science Institute, Ecole Polytechnique Fédérale of Lausanne (EPFL)

Professor Molinari received a BS and MS in mechanical engineering from the University of Technology of Compiègne in 1997. He received a MS in 1997 and a PhD in 2001, both in aeronautics, from the California Institute of Technology. From 2000 to 2005, he was assistant professor in mechanical engineering at Johns Hopkins University. He was then associate research professor from 2006 to 2011 at the same institution. From 2005 to 2007, he was also a professor at École Normale Supérieure Cachan in mechanics and held a teaching associate position at the École Polytechnique in Paris from 2006 to 2009. Molinari started his tenure at EPFL in 2007 as associate professor in structural mechanics in the School of Architecture, Civil and Environmental Engineering, and was promoted to full professor in 2012. He is the director of the Computational Solid Mechanics Laboratory at EPFL. He was director of the EPFL Civil Engineering Institute from 2013 to 2017, and has a joint appointment in the Materials Science institute. Molinari was elected in October 2019 to the Research Council of the Swiss National Science Foundation in mathematics, natural and engineering sciences. He is the chair of the Swiss Community for Computational Methods in Applied Sciences. Molinari is co-editor in chief for Elsevier's journal Mechanics of Materials.

Design is a process that proceeds from a desired overall outcome to the specifications for the individual components that enable the outcome. For materials science, design is a major challenge because it requires us to invert the typical modeling approach, which starts from microscale components in order to predict macroscale behavior. For granular matter additional complications arise from the fact that these materials are inherently disordered and typically far from equilibrium.

This talk will discuss recent progress in tackling this inverse problem and show how concepts from artificial evolution make it possible to identify with high efficiency particle-scale parameters that produce targeted macroscale behavior. In particular, we show how one can find particle shapes that are optimized for specific desired outcomes, such as low porosity or adaptive compliance under compression. The discussion will be embedded in the context of emergent applications for designed granular matter ranging from soft robotics to aleatory architecture.

About Heinrich Jaeger 
Sewell Avery Distinguished Service Professor of Physics
University of Chicago

Heinrich Jaeger received his PhD in physics in 1987 from the University of Minnesota and has been on the faculty at the University of Chicago since 1991, directing the Chicago Materials Research Center from 2001-06 and the James Franck Institute from 2007-10. Jaeger’s current research focuses on self-assembled nanoparticle-based structures, on the rheology of concentrated particle suspensions, and on studies of the packing and flow properties of dry granular materials.

Achieving enhanced energy dissipation is a daunting challenge for science and technology. In this talk, I will discuss how to achieve designer dissipation in compliant metamaterials. First I will discuss how to design metamaterials with multiple deformation pathways. In particular, I will describe how the combinatorics and machine learning can help to explore the design space efficiently. Second, I will discuss how these multiple deformation pathways can be controlled with dissipation and gain. Specifically I will illustrate: (i) how viscoelasticity can be used to achieve strain-rate dependent Poisson’s ratio and texture as well as enhanced vibration damping; (ii) how non-reciprocal gain can be used to create unidirectional wave propagation and unusual responses to impacts.

Our ability to improve more than one mechanical property in most engineering materials has been somewhat limited in the past by the inherent inverse relation between these desired properties often found in man-made materials. On the other side, Nature has evolved efficient strategies to synthesize materials that often exhibit exceptional mechanical properties that significantly break those trade-offs. In fact, most biological composite materials achieve higher toughness without sacrificing stiffness and strength in comparison with typical engineering material. Interrogating how Nature employs these strategies and decoding the structure-function relationship of these materials has opened up a new set of concepts in materials engineering. Considering the current progress in material synthesis and manufacturing, these new concepts have converged to the field of architectured materials.  In this talk, I will describe some interesting mechanics problems that we encountered as we studied some extraordinary species, and how we can translate these lessons learned to architectured materials. This includes a review of an interesting crack twisting mechanisms found in the helicoidal architecture of the dactyl club of the Mantis Shrimp and some new competing mechanisms that we recently found in the exoskeleton of the diabolical ironclad beetle, a terrestrial beetle that is well known for its high compressive strength, far beyond any other beetle identified to date. These naturally-occurring high-performance structures have been shown to be very efficient at promoting delocalization of damage and, therefore, at avoiding catastrophic failure.

Pablo D. Zavattieri, PhD
Lyles School of Civil Engineering, Purdue University
Dr. Pablo Zavattieri is the Jerry M. and Lynda T. Engelhardt Professor in Civil Engineering at Purdue University. Zavattieri received his BS/MS degrees in Nuclear Engineering from the Balseiro Institute (Argentina) and PhD in Aeronautics and Astronautics Engineering from Purdue University. He worked at the General Motors Research and Development Center as a staff researcher for 9 years, where he led research activities in the general areas of computational solid mechanics, smart and biomimetic materials. His current research lies at the interface between solid mechanics and materials engineering. He has focused on the fundamental aspects of how Nature uses elegant and efficient ways to make remarkable materials and their translation to engineering materials. He has contributed to the area of biomimetic materials by investigating the structure-function relationship of naturally-occurring high-performance materials at multiple length-scales, combining state-of-the-art computational techniques and experiments to characterize the properties.

The monitoring of the condition of structural systems operating under diverse dynamic loads involves the tasks of simulation (forward engineering), identification (inverse engineering) and maintenance/control actions. The efficient and successful implementation of these tasks is however non-trivial, due to the ever-changing nature of these systems, the variability in their interactive environments, and the polymorphic uncertainties involved. Structural Health Monitoring (SHM) attempts to tackle these challenges by exploiting information stemming from sensor networks. SHM comprises a hierarchy across levels of increasing complexity aiming to i) detect damage, ii) localize and iii) quantify damage, and iv) finally offer a prognosis over the system's residual life. When considering higher levels in this hierarchy, including damage assessment and even performance prognosis, purely data-driven methods are found to be lacking. For higher-level SHM tasks, or for furnishing a digital twin of a monitored structure,  it is necessary to integrate the knowledge stemming from physics-based representations, relying on the underlying mechanics. This talk discusses implementation of such a hybrid approach to SHM for tackling the aforementioned challenges. Among other topics, we will discuss the potential and limitations of purely data-driven schemes, and the benefits stemming from infusion of data with reduced order structural mechanics models, in support of diagnostics and prognostics for engineered systems.

Eleni Chatzi, PhD
Department of Civil, Environmental and Geomatic Engineering, ETH Zurich
Eleni Chatzi received her PhD (2010) from the Department of Civil Engineering and Engineering Mechanics at Columbia University, New York. In 2010, she proceeded to obtain an Assistant Professor position at ETH Zurich, where she currently serves as Associate Professor and Chair of Structural Mechanics and Monitoring at the Department of Civil, Environmental and Geomatic Engineering. Her research interests include the fields of Structural Health Monitoring (SHM) and structural dynamics, nonlinear system identification, and intelligent life-cycle assessment for engineered systems. She is an author of over 200 papers in peer-reviewed journals and conference proceedings, and further serves as an editor for international journals in the domains of Dynamics and SHM. She is currently leading the ERC Starting Grant WINDMIL on the topic of "Smart Monitoring, Inspection and Life-Cycle Assessment of Wind Turbines". Her work in the domain of self-aware infrastructure was recognized with the 2020 Walter L. Huber Research prize, awarded by the American Society of Civil Engineers (ASCE).

I will show two examples that illustrate the critical role that geometry plays in the mechanics of thin sheets. I will first show several experiments on fracture of thin sheet, a phenomenon that we experiment every day with paper or plastic packaging but that is very challenging to fracture mechanics. In a second part, I will explain how one can use geometry to engineer internal stresses in thin sheets and program shape-changes in architected active plates. These results have implications in the development of easy-opening packages and in soft robotics.

Benoit Roman, PhD
Benoit Roman (CNRS senior research fellow) is working in Paris on the mechanics of thin sheets: how fracture propagates in thin sheet (tearing), how they crumple, how they may be deformed by droplet (elasto-capillary effects) and how it is possible to activate them (shape-morphing).

Random fiber networks are present in many soft biological and man-made materials. Examples from the living world include the cellular cytoskeleton, the extracellular matrix and various types of connective tissue. Examples from the non-living world include paper, rubber, gels, non-wovens, insulation and hygiene consumer products. All these materials contain a random fiber network as their main structural component are united under the name of ‘network materials.’ We study the relationship between their microstructure and the mechanical properties, with emphasis on identifying regimes in which large changes of the system scale behavior are triggered by small changes in system parameters. The talk will review multiple aspects of this relationship for densely cross-linked, sparsely cross-linked and non-crosslinked networks. The question “how to reinforce soft network materials” will be addressed in the second part of the talk. To this end, networks made from fibers of different properties (reinforcement with fibers) and networks made from a single type of fiber and containing particulate inclusions (reinforcement with nanoparticles) will be discussed. Composite networks are used in a broad range of applications, from paper to tissue engineering, and the results discussed here are relevant for all these applications.

Catalin Picu, PhD
Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute
Prof. Picu received his PhD degree from Dartmouth College and spent two years as Research Associate at Brown University. He joined the Department of Mechanical, Aerospace and Nuclear Engineering at Rensselaer Polytechnic Institute in 1998, where he is now Professor and Associate Head. He is the author or co-author of two books, co-editor of two edited books, author of 18 book chapters and 200 journal articles. His research focuses on mechanics of materials, and in particular, on understanding the macroscopic material behavior based on physics taking place on multiple scales. He is a Fellow of ASME and Doctor Honoris Causa of the Polytechnic University of Bucharest, Romania.